It's around the perimeter for me (just opinion/personal preference) for reasons:
(1) Curves viewed from the inside appear less sharp to the mind's-eye than do curves viewed from the outside.
(2) I don't like spaghetti-bowls and sans that the maximum route-mileage is obtained by using the perimeter.
(3) Around-the-perimeter minimizes the real estate required for isles and thus maximizes the square footage available for the railroad.
(4) I want a model railroad to go from one place to another and for me that works best with around-the-perimeter because the sight-line is limited and keeps changing as the train moves along.
Lew